The 3 Numbers on Fertilizers Explained


The three numbers on fertilizer are the N-P-K ratio, which stands for nitrogen, phosphorus, and potassium. These three nutrients have separate functions and are essential to a plant’s growth and health. Fertilizers can be organic or synthetic, and come in a dry or liquid form.

Feeding your plants is an essential routine to care for them. Plant fertilizers increase a plant’s fertility and encourage growth. Fertlizers are either plant- or animal-based (organic) or man-made chemicals (inorganic), and some are water-soluble while others are dry granular. It can be confusing when it comes to choosing the right fertilizer for different plants. That’s why it is essential to find out what nutrients plants need and in which proportions. 

What Does Each Number Mean? The three numbers on the label indicate the ratio of each nutrient, or N-P-K ratio. The first number is for nitrogen (N), which increases leaf production and makes plants greener. Plants that are almost all leaves need a lot of nitrogen. The second is for phosphorus (P), which promotes root development, fruiting and flowering, and the third is for potassium (K), also known as potash which helps the plant fight off diseases, promotes hardiness and is essential to the metabolic process. Different amounts of nitrogen, phosphorus, and potassium will impact the growth of a plant in different ways. 

Each number represents the weight percentage of each nutrient in the package; for example, if a plant fertilizer contains a 10-15-10 ratio, this means it has 10 percent nitrogen, 15 percent phosphorus, and 10 percent potassium. The remaining 65 percent is a combination of other micronutrients such as iron, zinc, boron, and copper.

A higher concentration of any of the three nutrients will impact a plant’s development. High amounts of nitrogen will cause greater leafy growth, high phosphorus will cause more fruiting or blooming, and high potassium will cause greater hardiness. Fertilizers with higher ratios are more concentrated than those with lower ratios. High numbers need diluting with more water before applying to plants.

Fertilizer Types – Choosing between wet and dry is more of a preference. Granular fertilizers are mostly used for outdoor plants. They are generally cheaper and have a longer shelf life. For indoor plants or small container plants outside, liquid fertilizers are recommended. Diluted with water, they can get down to where the roots are rapidly.

Organic vs. Inorganic – Organic fertilizers work in a slow and natural way, while synthetic fertilizers work rapidly. Synthetic fertilizers tend to be more concentrated and need to be diluted. Organic fertilizers are made from natural ingredients, are less concentrated and take longer to break down once applied. This helps avoid over-fertilizing and provides steady nutrients over a longer period of time.

For gardens with edible crops, granular or dry fertilizer should be mixed in with the soil before planting in the spring. For gardens with perennial flowers, fertilize before the spring growing season. Over-fertilizing plants is usually worse than choosing the wrong numbers since over-fertilization can cause the plants to burn and leave them weak. If the plant’s requirements are different than what is indicated on the fertilizer numbers, plants will usually show signs.

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